Overvoltage protection for alternating current generators



April 27, 1954 H. 5 JAMES 2,677,076

OVERVOLTAGE PROTECTION FOR KETERNATING CURRENT GENERATORS Filed Dec. 8, 1951 WITNESSES:

OFF 40 a INVENTOR Homer B. James BY ATTOR EY Patented Apr. 27, 1954 OVERVOLTAGE PROTECTION FOR ALTER- NATING CURRENT GENERATORS Homer B. James, Lima, Ohio, assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application December 8, 1951, Serial No. 260,592

14 Claims. 1

The present invention relates to the control and protection of alternating-current generators, and more particularly to a control circuit for provid ing overvoltage protection for such generators.

While the usefulness of the present invention is not restricted to any specific type of generator, it is especially suitable for providing overvoltage protection for alternating-current generators intended for operation over a wide speed range, such as are used in variable-frequency alternatingcurrent aircraft electrical systems. Such systems are often used on aircraft for supplying loads which are not aifected by frequency variation, such as heating and lighting loads, since aircraft generators are usually driven from a main engine of the airplane and the use of a variable-frequency system permits elimination of the constant-speed drive which is required for a constant-frequency system, thus making possible a considerable saving in weight.

Protection against abnormal conditions must, of course, be provided in such systems, and one of the most serious conditions is overvoltage of the generator, which may be very damaging both to the generator itself and to the loads connected to the system. In providing overvoltage protection for wide-speed-range alternators, however, certain problems are encountered which are not present, or are less serious, in other types of machines. When field excitation is applied to such a generator to bring it up to voltage, especially when it is running at relatively high speed, there is a transient overshoot of the voltage due to the inherent delays in the voltage regulator. That is, the voltage will rise above the desired voltage for which the regulator is set and then drop back to the proper value. This transient overvoltage may be quite severe, but it can be tolerated as it is of very brief duration, and it is necessary to prevent operation of the overvoltage protection on this initial transient overvoltage, which would prevent putting the generator into use. This could be done, of course, by providing a suitable time delay in the overvoltage relay, but this is not a satisfactory solution of the problem, since such a time delay would impair the protective characteristics desired during normal operation.

Another problem in the protection of widespeed-range machines is that such machines usually have a relatively high residual voltage, which may be as high as 50% of the normal rated voltago. That is, when the machine is running at high speed with no field excitation, the generated voltage due to the residual magnetism of the magnetic circuit may be as high as 50 of the normal rated voltage. Overvoltage protective relays are normally designed to have a relatively low dropout voltage, so that when utilized with a machine having such a high residual voltage, the relay may be held in its actuated position by the re sidual voltage after it has once operated. This would make it impossible to restore the machine to service after an overvoltage condition has occurred without actually bringing the machine to rest to allow the relay to drop out, which is usually impractical in the case of aircraft generators since they are directly driven by a main engine of the airplane.

The principal object of the present invention is to provide a control and protective system for an alternating-current generator which will deenergize the generator field in response to overvoltage, but which will not operate on the transient overvoltage occurring when the field is initially energized.

Another object of the invention is to provide a control and protective system for alternatingcurrent generators utilizing an overvoltage relay for effecting deenergization of the generator field in response to overvoltage, and in which means are provided for temporarily making the overvoltage relay inoperative when the generator field is deenergized and subsequently reenergized, so that the relay positively drops out after operation and is prevented from operating during the initial build-up transient.

A further object of the invention is to provide a control and protective system for alternatingcurrent generators in which the field of the generator is deenergized in response to overvoltage and which is trip-free, so that the field cannot again be energized after operation of the overvoltage protection until the system has been manually reset.

More specifically, the invention provides a control and protective system for an alternatingcurrent generator having an overvoltage relay which effects deenergization of the generator field in response to overvoltage, and which also includes a time-delay relay for interrupting the sensing circuit of the overvoltage relay when the field is deenergized, to insure that the relay will drop out, and which reestablishes the sensing circuit with sufficient time delay, when the field is reenergized, to prevent operation of the overvoltage relay on the initial transient overvoltage. The system also includes a lookout relay which controls the energizing circuit of a field relay for effecting connection of the generator field to the source of excitation, the lockout relay being arranged to prey t reenergization of the field relay after operation the overvoltage relay until the system has been manually reset, so that the systern electrically trip free.

The invention will he more fully derstood from the following detailed description, talren in connection with the accompanying drav the single figure or" which is a schematic wiring diagram showing an illustrative embodiment of the invention.

Ihe invention is shown in the drawing bodied in a control and protective system for an alternatingwurrent generator l. lhe generator i is shown as a single-phase generator having an armature member 2 and a field winding 3. One terminal of the armature 22 is connected to a load bus and the other uerininal is connected to ground at 5. One terminal of the field winding is connected to a suitable SOl or direct-current excitation by of a field relay and the other terminal connected to ground, also at The system is shown as utilizing a ground return for both the al ernating and direct current circuits, since this is preferable for aircraft systems because of the saving in weight, but it Jill be apparent that separate wire return circuits might be used if desired. In the illustrated embodiment of the invention, direct-current excitation for the field winding 3 is provided from a direct-current bus '1, which supplied from any suitable source of direct current, shown diagrammatically a battery 3, on inal of which is connected to the bus the other terminal to ground at 9. While the invention is shown applied to a single-phase generator on cited from a direct-current bus, it will he parent that it is not limited to par icular type of generating system, but may be applied to any type of alternating current generator with any suitable source of excitation.

The output voltage of the alternator l is con trolled by a voltage regulator ill, which is shown as a carbon pile regulator having a carbon pil ll connected in series with the field winding to control. the hold current. The resistance of the carbon pile i! is varied oy varying the sure applied to it by a pressure device i2 of any suitable type, which is controlled by coil i3 acting in opposition to a spring The coil i3 is e ergized in response to the volts of the alter? or i, and is shown as being energized through. a single-phase rectifier bri ge connected across the generator voltage h the load pus Al and ground 15. A voltage justing rheostat ll may be connected in series with the rectifier is to adjust the generator voltage.

Overvoltage protection is provided by means of an overvoltage relay H which has an operating coil it and a normally closed contact Th relay i3 is adjusted to pick up and open its contact 253 when the voltage applied to the operating coil it exceeds a predetermined value corresponding to the maximum permissible voltage of the generator l. The relay i8 referably has inverse time delay characteristics so as to oper ate more rapidly on higher overvoltages. The coil is is energized through a single-phase rec tifier bridge El which is connected across the generator voltage in a sensing circuit which extends from the hue l through a conductor ill, a volt age adjusting rheostat 22, a resistor 23, the. rectiller 2i, and the contact 24 of a time-delay relay 25 to ground 25.

The field relay referred to above, has an operating coil 28 adapted to be energized from the direct-current bus '5, and has a normally open contact 29 and a normally closed contact 38. The normally open contact 29 is connected in series with the generator field winding 3 to connect the field winding to the direct-current bus '7 when the field relay 6 is energized. The normally closed contact is connected in series with the operating coil 32 of the time-delay relay 25 to energize the relay 25 from the directcurrent bus '5 when the field relay 5 is deener giaed. The time-delay relay 25 is adapted to open its contact substantially instantaneously when the relay is energized and to close its contact M with a definite time delay when the relay is deenergised, any suitable time delay mechanism, indicated diagrammatically at 32, being provided for that purpose. As indicated above, the contact fill is connected in the sensing circuit of the overvoltage relay iii to control the over-voltage relay, so that the relay i3 is made temporarily inoperative when the relay 2% is energized.

A lockout relay is also provided having an operating coil 3- 1 which is connected to the direct-current bus in series with the contact is oi he over-voltage relay it. The lockout relay J. as contacts and til which are closed when the relay is enel d and a contact 37 which closes when the relay is deenergized. The contact to is connected to control the energization oi the field relay as hereinafter described.

A manual switch is provided for controlling the operation of the system. The switch 38 has an on contact as and an off and reset contact ill, and is adapted to connect these contacts alternatively to the direct-current bus '5. The on contact is connected to the operating coil 28 or" the field relay ll, while the contact 40 is connected to control the lockout relay The operation of this system is as follows. Assume that the generator i is running at normal speed and it is desired to apply excitation and bring the generator up to voltage. The switch 33 under these circumstances, when the generator is not in service, will be on the contact it, as shown in the drawing. The field relay 6 is cleanergizecl so that the contact 2c is open, while the contact is closed and connects the coil 31 of the time-delay relay ll directly between the direct-current hus l and ground 23. The relay 25 is, therefore, energized and its contact is is open. The overvoltage relay is is deenergized, and its contact 2% is closed, since the sensing circuit for the relay 4% is interrupted by the contact With the switch 38 on the contact 49, the lockout elay is energized from the direct-current bus 'l through the conductor 4|, the contact 20 of the overvoltage relay l8, and the col 34 of the lookout relay 33 to ground 42. The lockout relay 53 is, therefore, in the energized position shown with the contacts 35 and 36 closed. The contact 35 of the lookout relay completes a sealing circuit for the relay from the bus 1 through conductor 33, contact contact illi of the relay l8 and coil 3 to ground. Thus the relay 33 is held in its energized position after it has once been energized by placing the switch 38 on the contact 48.

If it is now desired to energize the field winding 3 to bring the generator I up to voltage, the

manual switch is moved to the on contact 39..

This completes an energizing circuit for the field relay 6 from the direct-current bus 1 through the contact 39, relay coil 28, conductor 44, and contact of the lockout relay 33 to ground 42. The contact 35 remains closed when the switch 38 is operated since the relay '33 is held in its energized position by the sealing circuit through the contact 35. When the field relay 6 is thus energized by placing the switch 38 on the contact 39, it picks up and closes its contact 29 to connect the field winding 3 to the direct-current bus 1 to apply excitation to the generator. Simultaneously the field relay contact 36 is opened and deenergizes the time-delay relay 25, which drops out and closes its contact 24 with a suitable time delay which may, for example, be of the order of one-tenth second. This completes the sensing circuit of the overvoltage relay l8, so that it is connected to be responsive to the voltage of the generator 4, but because of the time delay in the relay 25, the sensing circuit is not completed until after the transient overshoot of the generator voltage which occurs when the field is initially energized has passed, so that the overvoltage relay i8 is prevented from operating on this transient overvoltage.

The system is now in its normal operating condition. If the generator voltage exceeds the value for which the overvoltage relay I8 is set, for any reason, the relay 18 picks up and opens its contact 20. This interrupts the energizing circuit for the coil 34 of the lockout relay 33 so that the lockout relay drops out and opens its contacts 35 and 36 and closes its contact 3?. The contact 31 connects an indicating lamp to the bus "i through the conductor 63 to indicate operation of the relay. The contact as interrupts the energizing circuit of the field relay coil 28 so that the field relay 8 drops out and opens its contact 29, thus disconnecting the field winding 3 from the bus I and removing excitation from the generator. At the same time, the contact 30 of the field relay 6 closes and connects the coil 3| of the time-delay relay 25 to the bus 7, energizing the relay 25 to cause it to open its contact 24 substantially instantaneously. This interrupts the sensing circuit of the overvoltage relay I8 so that it drops out and recloses the contact 20. In this way positive dropout of the relay i8 is insured regardless of the magnitude of the residual voltage of the generator I, which might be high enough to hold the relay [8 in its actuated position, which would make it impossible to again energize the field winding 3. When the lockout relay 33 opens its contact 36, the sealing circuit which maintained its energization is interrupted, so that when the relay l8 recloses its contact 20, the lockout relay 33 is not energized. Thus the system is electrically trip-free, since the field relay 6 cannot be reenergized after operation or the protective relay i8 until the system is manually reset.

When it is desired to reapply excitation to the generator 5, the switch 38 is manually moved to the reset contact 48, which energizes the lockout relay 33, and the field winding 3 can then be energized by moving the switch 33 to the on contact 39, the operation being as previously described. In normal operation, if it is desired to remove excitation from the generator I, it is only necessary to move the switch 38 from the contact 38 to the contact 48, which interrupts the energizing circuit of the field relay 6 and causes it to disconnect the field winding 3 from the bus I. This operation does not, however, deenergize the lockout relay 33, so that the system remains in the condition shown in the drawing, ready to re-' energize the field 3 by operation of the switch 38. It will be noted that the indicating light 45 is not lighted when the field winding 3 is deenergized manually, since the lockout relay 33 is not deenergized and the contact 31 remains open. Thus, the light 45 is lighted only when the system is tripped by operation of the overvoltage relay l8 and gives a reliable indication that an overvoltage condition has occurred.

It should now be apparent that a control and protective system has been provided for alternating current generators which provides positive and reliable protection against generator overvoltage, but which will not operate on the transient overvoltage which occurs during the initial buildup period, and which is electrically trip-free so that excitation cannot be reapplied to the generator after operation of the protective system until the system is manually reset. The time-delay relay 25 prevents operation on the initial transient overvoltage by temporarily rendering the overvoltage relay l8 inoperative during the initial buildup period but restores it to operativeness immediately thereafter. The operation of the time-delay relay to interrupt the sensing circuit of the overvoltage relay also insures that the overvoltage relay will drop out after operation regardless of the value of residual voltage of the generator. The lockout relay 33 provides the desired trip-free operation and also makes it possible to utilize the indicating light 45 as a reliable indication that an overvoltage has occurred and caused tripping of the field relay. Thus a very desirable and effective control and protective system is provided which is espectially adapted for use with machines designed for operation over a wide speed range, although it will be apparent that its usefulness is not limited to this particular type of machine, or to the particular type of generator and excitation system shown in the drawing. An illustrative embodiment of the invention has been shown and described, but it will be obvious that various modifications and other embodiments are possible within the scope of the invention, and it is to be understood, therefore, that the invention is not limited to the specific details and circuit connections shown, but in its broadest aspects it includes all equivalent embodiments which come within the scope of the appended claims.

I claim as my invention:

1. A control system for an alternating-current generator having a field winding, said control system including field relay means for controlling the connection of said field winding to a source of direct current excitation, means for energizing said field relay means to effect connection of the field winding to said source, voltage-responsive relay means for effecting deenergization of the field relay means in response to generator voltage in excess of a predetermined value, and relay means adapted to render said voltage-responsive relay means inoperative upon deepergization of the field relay means, said last-mentioned relay means being adapted to restore the voltage-responsive relay means to operativeness with a predetermined time delay upon energization of the field relay means.

2. A control system for an alternating-currentgenerator having a field winding, said control system including field relay means for controlling the connection of said field winding to a source of direct-current excitation, means for energizing said field relay means to eitect connection of the field winding to said source, voltage-reaevaove sponsive relay means connected to respond to the voltage of the alternating-current generator and adapted to effect deenergization of the field relay means when the generator voltage exceeds a predetermined Value, and relay means adapted to interrupt said connection of the voltagere sponsive relay means upon deenergization of the field relay means, said last-mentioned relay means being adapted to reestablish said connection with a predetermined time delay upon energization of the field relay means.

3. A control system for an alternating-current generator having a field winding, said control system including field relay means for controlling the connection of said field winding to a source of direct-current excitation, means for energizing said field relay means to effect connection of the field winding to said source, voltage-responsive relay means having an operating coil connected across the voltage of the alternatingcurrent generator and being adapted to efiiect deenergization of the field relay means when the generator voltage exceeds a predetermined value, and relay means having a contact connected in series with the operating coil of the voltageresponsive relay means, the field relay means being adapted to effect actuation of said lastmentioned relay means to open its contact when the field relay means is deenergized, the lastmentioned relay means being adapted to reclose its contact with a predetermined time delay when the field relay is energized.

4. A control system for an alternating-current generator having a field winding, said control system including field relay means for controlling the connection of said field winding to a source of direct-current excitation, means for energizing said field relay means to efiect connection of the field winding to source, voltage-responsive relay means having an operating coil connected across the voltage of the alternatingcurrent generator and being adapted to effect deencrgization of the field relay means when the generator voltage exceeds a predetermined value, and relay means having a contact connected in series with the operating coil of the voltage-responsive relay means, the field relay means including means for controlling the energization of said last-mentioned relay means and being adapted to effect energization of said relay means to open. its contact when the field relay means is deenergized, the last-mentioned relay means being adapted to reclose its contact with a predetermined. time delay when it is deenergized by energization of the field relay means.

5. A control system. for an alternating-current generator having a field winding, said control system including field relay means for effecting connection of said field winding to a source of direet current excitation, relay means for controlling the energization of said field relay means, means responsive to the voltage of the alternating-current generator for controlling said relay means to efiect deenergization of the field relay means when said voltage exceeds a predetermined value, and means for rendering said voltageresponsive means inoperative upon deenergization of the field relay means and for restoring the voltage-rcsponsive means to operativeness with a predetermined time delay upon energization of the field relay means.

6. A control system for an alternating-current generator having a field winding, said control system including field relay means for effecting connection of said field winding to a source of direct-current excitation, relay means for controlling the energization of said field relay means, means responsive to the voltage of the alternat lug-current generator for controlling said relay means to effect deenergization of the field relay means when said voltage exceeds a predetermined value, means for rendering said voltage-responsive means inoperative upon deenergization of the field relay means and for restoring the voltage-responsive means to operativeness with a predetermined time delay upon energization of the field relay means, and manual means for effecting energization of the relay means and for completing an energizing circuit for the field relay means.

'7. A control system for an alternating-current generator having a field winding, said control system including field relay means for effecting connection of said field winding to a source of direct-current excitation, relay means for controlling the energization of said field relay means, voltage-responsive means connected to respond to the voltage of the alternating-current generator for controlling said relay means to effect deenergization of the field relay means when said voltage exceeds a predetermined value, and means for interrupting said connection of the voltage-responsive means upon deenergization of the field relay means and for reestablishing the connection with a predetermined time delay upon energiaation of the field relay means.

3. A control ,ystem for an alternating-current generator having a field winding, said control system including field relay means for effecting connection of said field winding to a source of direct-current excitation, relay means for controlling the cnergization of said field relay means, a voltage-responsive relay having an operating coil connected to respond to the voltage of the alternating-current generator, the voltageresponsive relay being adapted to control said relay means to effect deenergization of the field relay means when said voltage exceeds a predetermined value, and a time-delay relay having a contact connected in series with the operating coil of the voltage-responsive relay, the field relay means being adapted to efieet actuation of the time-delay relay to cause it to open its contact upon deenergization of the field relay means, the time-delay relay being adapted to reclose its contact with a predetermined time delay upon energization of the field relay means.

9. A control system for an alternating-current generator having a field winding, said control system including field relay means for effecting connection of said field winding to a source of direct-current excitation, relay means for controlling the energization of said field relay means, a voltage-responsive relay having an operating coil connected to respond to the voltage of the alternating-current generator, the voltageresponsive relay being adapted to control said relay means to efiect deenergization of the field relay means when said voltage exceeds a predetermined value, a time-delay relay having a contact connected in series with the operating coil of the voltage-responsive relay, the field relay means being adapted to effect actuation of the time-delay relay to cause it to open its contact upon deenergization of the field relay means, the time-delay relay being adapted to reclose its contact with a predetermined time delay upon energization of the field relay means, and manual means for effecting energization of the relay means which controls the field relay means and for completing an energizing circuit for the field relay means.

10. A control system for an alternating-current generator having a field winding, said control system including field relay means for effecting connection of said field winding to a source of direct-current excitation, a lockout relay connected to control an energizing circuit for said field relay means, a voltage-responsive relay responsive to the voltage of the alternating-current generator, said voltage-responsive relay being connected to control an energizing circuit for the lookout relay and being adapted to interrupt said energizing circuit to cause the lookout relay to efiect deenergization of the field relay means when said voltage exceeds a predetermined value, and means for rendering said voltage-responsive means inoperative upon deenergization of the field relay means and for restoring the voltageresponsive means to operativeness with a predetermined time delay upon energization of the field relay means.

11. A control system for an alternating-current generator having a field Winding, said control system including field relay means for effecting connection of said field winding to a source of direct-current excitation, a lockout relay connected to control an energizing circuit for said field relay means, a voltage-responsive relay responsive to the voltage of the alternating-current generator, said voltage-responsive relay being connected to control an energizing circuit for the lookout relay and being adapted to interrupt said energizing circuit to cause the lookout relay to effect deenergization of the field relay means when said voltage exceeds a predetermined value, means for rendering said voltage-responsive means inoperative upon deenergization of the field relay means and for restoring the voltageresponsive mean to operativeness with a predetermined time delay upon energization of the field relay means, and manual means for effecting energization of the lookout relay and for completing the energizing circuit of the field relay means.

'12. A control system for an alternating-current generator having a field winding, said control system including a field relay for effecting con nection of the field winding to a source of directcurrent excitation, a lockout relay having a contact connected in an energizing circuit for said field relay, manual means for completing an energizing circuit for the lookout relay to cause it to close its contact and for completing the energizing circuit of the field relay to cause it to connect the field winding to the direct-current source, the lookout relay also having means for maintaining its energizing circuit, a voltageresponsive'relay having a contact connected in the energizing circuit of the lookout relay, said voltage-responsive relay being adapted to open its contact when the voltage of the alternatingcurrent generator exceeds a predetermined value, and means for rendering said voltage-responsive 10 means inoperative upon deenergization of the field relay means and for restoring the voltageresponsive means to operativeness with a predetermined time delay upon energization of the field relay means.

13. A control system for an alternating-current generator having a field winding, said control system including a field relay for effecting connection of the field winding to a source of directcurrent excitation, a lockout relay having a con tact connected in an energizing circuit for said field relay, manual means for completing an energizing circuit for the lockout relay to cause it to close its contact and for completing the energizing circuit of the field relay to cause it to connect the field winding to the direct-current source, the lockout relay also having means for maintaining its energizing circuit, a voltage-responsive relay having a contact connected in the energizing cir cult of the lockout relay, said voltage-responsive relay being connected to respond to the voltage of the alternating-current generator and being adapted to open its contact when said voltage exceeds a predetermined value, and means for interrupting said connection of the voltage-responsive means upon deenergization oi the field relay means and for reestablishing the connection with a predetermined time delay upon energization of the field relay means.

14. A control system for an alternating-current generator having a field Winding, said control system including a field relay for efiecting connection of the field winding to a source of directcurrent excitation, a lookout relay having a contact connected in an energizing circuit for said field relay, manual means for completing an energizing circuit for the lookout relay to cause it to close its contact and for completing the energizing circuit of the field relay to cause it to connect the field winding to the direct-current source, the lockout relay also having means for maintaining its energizing circuit, a voltageresponsive relay having a contact connected in the energizing circuit of the lookout relay, said voltage-responsive relay having an operating coil connected to respond to the voltage of the alternating-current generator and being adapted to open its contact when said voltage exceeds a predetermined value, and a time-delay relay having a contact connected in series with the operating coil of the voltage-responsive relay, the field relay means being adapted to effect actuation of the time-delay relay to cause it to open its contact upon deenergization of the field relay means, the time-delay relay being adapted to reclose its contact with a predetermined time delay upon energization of the field relay means.

References Cited in the file Of this patent UNITED STATES PATENTS Number 

